Low slump concrete or cementitious mixtures are used in the construction industry in a variety of applications. Slip-form paving is one example where concrete is extruded in place without the use of stationary forms and the concrete must maintain its shape after it leaves the paving machine. Low slump mixtures, generally having slumps less than 3 inches, have been utilized for slip-form paving applications, because they provide some level of workability for placing the mixture while maintaining their shape after consolidation and extrusion. Other illustrative applications for low-slump mixtures include extruded in-place curb and gutter applications or sidewalk applications.
Each of these applications relies on vibration energy to facilitate placement and consolidation of the low-slump cementitious mixture. When vibration is used, the paste fraction of the mixture liquefies, thereby providing a temporary increase in workability to the otherwise low workability mixture facilitating proper consolidation. During consolidation there is an initial subsidence of the concrete due to the removal of larger air pockets, followed by a further release of smaller air voids of approximately 1 inch or less.
As the process continues, large aggregate particles re-orient themselves such that more optimal packing occurs. The result of proper consolidation is a more uniform, void free material ensuring that the designed strength and durability is achieved. Incomplete or improper consolidation can leave large voids or areas of honeycombing within the concrete leading to low compressive or flexural strengths, reduced bond to internal steel reinforcement, or weak, porous zones once setting and hardening has taken place. Due to the low workability of low slump mixtures, generally high levels of vibration energy are required to properly and fully consolidate the mixture. These high levels of vibration can lead to problems such as localized segregation or loss of entrained air due to over-vibration.
Typical slip-form pavers perform screeding, consolidating and initial finishing of concrete in-place as the paver machines move along the roadbed. The basic practice involves depositing a surplus of concrete in front of the paving machine where it is spread out by a horizontal auger, ensuring sufficient concrete depth at the sides of the paver. It is then struck off at the correct elevation and fed into a series of vibrators where it is consolidated. Establishment of the final dimensions of the slab and finishing is accomplished by extruding the mixture through horizontal and vertical forms at the rear of the paver. Concrete mixtures having very low slump (about 1″) are generally desired, as these mixtures tend to maintain square edges after extrusion. However, these mixtures are sometimes difficult to consistently produce. Batch to batch variations that result in slumps lower than designed can cause a slow down in production in order to ensure that adequate vibration time is provided to properly consolidation the concrete. Slumps that are too low can also cause the paver to rise up onto the surplus concrete in the front of the paver. This changes the elevation of the machine resulting in dips and valleys of the in-place concrete.
What is needed in the industry is an additive for low-slump cementitious mixtures that improves the low slump mixture's response to vibration, and dampens out differences due to minor fluctuations in initial slump. Improving the response to vibration would allow proper consolidation to be achieved with lower vibration energy, increased production or extrusion speed, and more consistent placement of low slump cementitious mixtures. Lower vibration levels will decrease the risk for over-vibration or disruption of the entrained air-void system.